1. Field of the Present Invention
The invention relates to methods that will attenuate or eliminate unwanted movement or electrostatic interference on the signal acquired from non-resistive contact sensors that are used exclusively or in combination with other sensors and the sensor data is utilized for detecting properties of an entity and entities (biological or otherwise). For biological entities the invention utilizes an electric field sensor or sensors for the measurement of the structural and functional characteristics of organs and other structures where the electric field sensor does not have resistive contact with the organism, conferring multiple advantages. More particularly, the invention relates to sensors, sensor housings, fastenings and sensor systems including devices and installations for assemblies for detecting structural and functional signatures associated with electric potentials that may detect a displacement signature within the geomagnetic field, and/or specific components and/or structures that are a component of that entity or entities. Specifically there is no resistive contact between the entity and the signal transduction component of the electric field sensor or sensors. Other sensor types may be added in to provide further information such as for the identification and elimination or attenuation of unwanted electrostatic or movement signal associated with the recording of non-resistive contact electric fields from that entity, in whatever state, such as during active or passive movement.
2. Background
Conventional electrodes act as a current transducer converting ionic currents into electronic ones so electrophysiological status can be assessed. The uses for this are many and broadly range from assessment of neural (EEG), and cardiac (ECG) and skeletal (EMG) muscle activity.
This approach requires conductive contact with the source and has inherent problems. The first of these is the requirement of clean skin exposure. This requirement may compromise continuous usability due to the effects of environmental contaminants, both on the skin and in the atmosphere; extremes of temperature and their resulting general effect on skin due to physiological reactions such as “goose bumps” and excessive sweating as well as other phenomena; and potential reactions to conductive materials. The process of preparing skin and securing a good conductive contact can also decrease compliance, especially in if intended for continuous day to day use. Furthermore, during exercise, the physicality can result in electrodes being displaced. The other issues include: shorting between electrodes, especially when placed in close proximity to each other; and charge transfer which has potential safety implications as well as the issue of the measurement process corrupting the signal.
The problems, outlined above, are solved by the use of capacitive electrodes (non-resistive contact sensors) as they acquire signal through capacitive coupling, not requiring resistive contact with the source. They provide many benefits, including the fact that no electrical contact is required (and so no skin preparation or conducting pads are necessary, and they can be readily moved or relocated to get an optimal signal), they can be miniaturized, they have very low power requirements, and they can be embodied as passive electric field sensors with the result that adjacent sensors do not interfere with each other.
The use of capacitive electrodes for electrophysiological monitoring is not a recent innovation, with Richardson describing it for acquisition of the cardiac signal in 1967 (see The insulated electrode: a pasteless electrocardiographic technique. Richardson P C. Proc. Annu Conf. on Engineering in Medicine and Biology 7: 9-15(1967)). This system was, however, flawed being prone to problems including poor signal to noise ratio, voltage drift, electrostatic discharge and parasitic capacitance. These are still problems with capacitive sensor technologies today. Many of those problems have been addressed, at least partially, however problems with electrostatic interference still plague this technology. Electrostatic interference is especially problematic during movement. Movement may lead to a variety of issues that may compromise continuous signal acquisition including: contact electrification between the body surface and the sensor electrode; charge build-up on the body resulting in baseline shift and potential saturation if occurs too rapidly; and movement of the sensor relative to the body that can also lead to baseline shift and saturation (railing).
The use of dry electrodes pressed into direct contact with the person may create triboelectric effects. That is, electrical charges created by sliding friction and pressure. Triboelectric effects of this nature may cause contact electrification where static charges may be delivered to the pick-up electrode. This static charge can produce a near direct current (DC) or very low frequency drift in sensor that may interfere with the physiological alternating current (AC) that is being measured or saturate the sensor causing railing, after which the sensor takes time to return to being able to produce a useful physiologically relevant output. If the electrode moves relative to the body, it will also pick up a geoelectric displacement signal. That is, the effect of the body, an electrically active structure, moving through the geoelectric field that is of the order of 100 Vm−1 will cause relative polarization of the sensor that will displace the baseline and may cause the sensor to saturate. An additional source of interference is that of clothing moving on the body. As clothing moves on the body then charge separation can occur when materials that are separated on the triboelectric series donate or receive electrons from each other. After a material becomes charged it may discharge onto the surface of where an electric potential may be being measured thereby interfering with signal acquisition. Cotton is a relative exception to this as it is essentially triboelectrically neutral, or does not accept or give up electrons, so charge separation tends not to occur.